This invention relates to part sizing or gaging devices for indicating part size deviation of a machined part or the like from a predetermined nominally ideal size.
In the course of machining precision parts it is customary to check the size of the part by comparing it to a part of nominally ideal or perfect size. One well-known device for accomplishing this is a vertical glass tube through which air is caused to flow in varying rates according to the size deviation of a part under inspection. The vertical air column supports an indicator float in a vertical position which is related to part size. Thus, a given float position may be equated to ideal or mean size while positions above and below the given position may be equated to part sizes which are over and under the mean value, respectively.
Several prior art devices have provided electronic implementations for such a gage. For example, in U.S. Pat. No. 4,038,756 to Smith et al, issued Aug. 2, 1977 and assigned to the same assignee as the present invention, a vertical column of light emitting diodes (LED's) is utilized to provide a visual indication of the part size under test by lighting an appropriate LED in the column. In this device the probe output signal when measuring an ideal part is utilized to zero the apparatus so that an LED in the middle of the display is lit. If the test part is larger or smaller than the ideal part, an LED is lit above or below the middle LED, respectively, and it is displaced from the middle LED a distance representing the degree of deviation. Manually slidable overlay elements on the display are positioned along the column to indicate the upper and lower limits of the degree of deviation that can be tolerated for a given part. Unfortunately, these overlays are susceptible to being tampered with or otherwise accidentally moved due to vibrations, etc.
Obviously, accuracy is of utmost importance in part sizing systems. The most widely used probes are of a variable reluctance type in which the position of the probe induces a corresponding voltage change in a winding surrounding the probe element. An excitation voltage applied to the windings is used to set up the necessary inductive coupling between the probe element and its windings. It is imperative that the excitation voltage remain constant, otherwise the probe output signal would reflect such a change and not provide a true indication of the size of the part under test as compared with an earlier measured part. One method of maintaining a constant excitation voltage is shown in copending U.S. patent application No. 898,831 to Juengel et al entitled "Multiple Probe Gage System", filed Apr. 21, 1978 and having the same assignee as the present invention. A feature of the present invention is to provide improved and more economical circuitry for maintaining a constant excitation voltage.